This application claims the priority of German patent document 199 40 026.1, filed Aug. 24, 1999, and PCT International Application No. PCT/EP00/07626, filed Aug. 4, 2000, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a power steering system having a hydraulic feedback arrangement that generates a reaction force at the steering column.
German patent document DE 41 12 735 C2 discloses such a power steering system, in which the steering reaction is generated as a function of the speed of the motor vehicle, and can be felt by the driver of the motor vehicle as a steering resistance. In the connecting line of this feedback device there is a throttle for adjusting the characteristics of the power assist.
In motor vehicles of the current 5 and 7 series of the assignee of this application, a power steering system, called xe2x80x9cServotronicxe2x80x9d, is installed as optional equipment. Contrary to the power steering systems such as is known from the German patent document DE 41 12 735 C2, in which the feedback is the result of the amount of power assist, these motor vehicles of the assignee have a separate feedback device, where a reaction force is applied to the steering shaft by way of a pressure drop at a series connection of hydraulic resistors.
In known steering systems high frequency pressure surges, which generate only a low volume flow, are not affected in principle by the hydraulic resistance. Consequently high frequency pressure fluctuations, generated, for example, when driving on rough roads, are passed largely unfiltered into the feedback device and induce steering wheel torque jolts over both the working cylinder of the power steering system and the hydraulic feedback device. Especially in the case of rack and pinion steering systems with their comparatively low mass moments of inertia, road excitations lead to a xe2x80x9cbumpinessxe2x80x9d in steering.
One object of the invention is to provide an improved power steering system of the type described above.
This and other objects and advantages are achieved by the power steering system according to the invention, in which the vibrating behavior of the liquid column in the connecting line of the feedback device is influenced as a function of the frequency of the vibrations. To this end, there is preferably a hydraulic low pass filter, which is effective to filter out jolts which have a frequency above that of typically introduced steering movements (claim 6).
In one embodiment of the invention, the filter includes a hydraulic capacitor, which is formed, for example, by a piston-type, bladder or diaphragm accumulator. Furthermore, the hydraulic capacitor can also be implemented by especially elastic walls of the oil-conducting chambers inside the steering system. Such a configuration can be achieved, for example, with a hose connection. The hydraulic capacitor can be combined with a hydraulic resistor (constrictor or throttle) in series connection, according to a preferred arrangement of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.